PubMed itself supports limited phrase search, sometimes reporting,
"Quoted phrase not found.", even when Google finds the phrase.
See
this email archive item
for further comments on the PubMed search.
Google Scholar below does a good job of harvesting papers on the web,
including references to them.
Also, check out CiteSeer which does citation analysis of papers it finds on the web.
BLIMP (Biomedical LIterature (and text) Mining Publications) has many BioNLP papers. Here's their search page. A useful search is simply one on the most recent year, e.g., "2006". (as of 10/06 "2005" returns 94 links to full papers or abstracts - a substantial number.)
Updated 10/28/2006